As an example of a device for forming a compound thin film such as one composed of an oxide or a nitride on a substrate, a reactive sputtering device 110 illustrated in FIG. 1 is known. The reactive sputtering device 110 includes a vacuum chamber 111, a metal target 113 provided inside the vacuum chamber 111 and a power supply 112 that supplies constant power to the metal target 113. A substrate 50 is arranged at a position facing the metal target 113 inside the vacuum chamber 111. The inside of the vacuum chamber 111 is evacuated using a vacuum pump 121, a non-reactive gas is introduced via a non-reactive gas introduction valve 116 and a reactive gas is introduced via a reactive gas introduction valve 115. The pressure inside the vacuum chamber 111 is maintained at a certain pressure by an automatic pressure adjustment valve 122 provided between the vacuum chamber 111 and the vacuum pump 121.
FIGS. 2 and 3 are diagrams for explaining the deposition mode in reactive sputtering and illustrate the relationship between the absolute value of a target voltage V applied to the metal target and a gas flow rate Q of a reactive gas introduced into the inside of the vacuum chamber. The flow rate of a non-reactive gas introduced into the inside of the vacuum chamber is constant. There are a metal mode M, a compound mode C and a transition mode T within the deposition mode in reactive sputtering.
FIG. 2 illustrates that the target voltage V changes as indicated by the solid line arrows when the gas flow rate Q is gradually increased, or the target voltage V changes as indicated by the broken line arrows when the gas flow rate Q is gradually decreased. Thus, the gas flow rate Q and the target voltage V in reactive sputtering have a hysteresis characteristic.
FIG. 3 illustrates the relationship between the gas flow rate Q and the target voltage V in the case where the gas flow rate Q is controlled in order to control the target voltage V to be constant. Although there are fluctuations in the gas flow rate Q and the target voltage V in reality, the gas flow rate Q and the target voltage V are illustrated as a curved line such as the deposition mode curve D1 by plotting average values of the gas flow rate Q and the target voltage V. When forming a thin film, sputtering is performed using any of deposition conditions located along the deposition mode curve D1.
The transition mode T along the deposition mode curve D1 is located between the metal mode M and the compound mode C and is a mode in which a compound thin film can be formed on the substrate 50 while maintaining an eroded portion of the metal target 113 in a metal state. Consequently, the transition mode T has both the strong points of having a deposition rate that is close to that of the metal mode M and of being able to form a compound thin film with a composition close to that in the compound mode C. However, the transition mode T also tends to easily change to the metal mode M or the compound mode C with a change in the discharge state inside the vacuum chamber. Therefore, in order to maintain the transition mode T when forming a thin film, impedance control in which the target voltage V is maintained constant is typically performed.
In addition, in embodiment 9 of Japanese Unexamined Patent Application Publication No. 2007-63623, a finding is disclosed regarding the relationship between a discharge voltage and a partial pressure of a reactive gas (O2 gas) inside a vacuum chamber. According to this finding, reactive sputtering using the transition mode T can be maintained by setting the discharge voltage and the partial pressure of the reactive gas so as to be within a certain range (plateau region).